It has become increasingly popular in recent years to perform geo-physical exploration in order to search for subterranean formations known to be indicative of the possible presence of gas or oil. In seismic exploration, an acoustic wave is propagated through the earth and is reflected from interfaces between subterranean layers. The time taken for the reflected wave to return to its source, or to a geophone placed nearby, is indicative of the depth to which the wave travelled before reflection and can therefore be used to derive an indication of the subterranean structure. If a number of parallel measurements are made, the subterranean structure can therefore be charted. Similar techniques may be used on land, or at sea, depending on where the exploration is is being carried out.
The development of useful pictures of the underground formation from geophonic records of the propagation or reflection of acoustic waves involves a great deal of skill, particularly inasmuch as the waves frequently must travel great distances before returning to a geophone. Therefore, they are very much susceptible to electronic and acoustic noise and must be very carefully processed in order to yield significant results.
Increasing the number of geophones "listening" to a given seismic impulse being reflected from subterranean formations increases the amount of data available to be processed. The prior art describes increasing the number of geophones, the number of data recording devices, and the resulting number of seismograms in seismic exploration. Numerous inventions have been made in order to permit such expansion of the data base available to the geophysicist for interpretation. Some of the expedients used are described in commonly assigned U.S. Pat. Nos. 4,016,531 and 4,084,151 as well as in commonly assigned co-pending application Ser. No. 118,299, filed Feb. 4, 1980, now U.S. Pat. No. 4,323,990. The goal of all three of the inventions described by these disclosures is to perform additional data processing on the data while it is still being stored on an ocean-going exploration vessel. This places the seismic data in a format in which it can be most efficiently transported to and processed on the land based computer which is designed to remove noise and to produce accurate pictures of subterranean formations for geophysical use. The better the condition of the data while still on board the vessel, the better the results will be from the land based computer. The present invention is concerned with means for improving these results and comprises a real-time monitor which gives an approximate picture of the data as it is being recorded. From this, the operator of the seismic exploration system can determine if the system is functioning properly, and can accordingly make corrections.
Seismic exploration for oil and gas frequently involves the use of a specially constructed seismic exploration vessel. This vessel produces acoustic impulses from compressed air guns which are towed by the vessel. These guns are filled with compressed air at very high pressure which is rapidly released into the water, thus sending a sonic wave through the water. This wave passes through the sea and through the sea bed and is partially reflected at the interface between each layer of differing density; thus, a large fraction of the wave is reflected at the sea bottom, and further fractions are reflected at the first interface between one type of layer under the sea and a second type, and so on until the wave is dissipated. Behind the boat is streamed a long cable having a large number of acoustic energy detectors streamed therefrom. These acoustic energy detectors may comprise piezoelectric hydrophones. These are, in turn, connected to circuitry for converting the analog wave forms ("traces") produced by the seismic energy detectors into digital data suitable for processing by computer to yield a picture of the subsea bed. In the preferred embodiment to be described more fully hereinafter, 208 hydrophone traces are used. The acoustic waveform represented by each trace is sampled every four milliseconds and used to produce a digital representation of its instantaneous amplitude. The words of digital data thus produced, it will be apparent, will be produced in the order of the time of sampling; that is, they will be serial by time of detection. It is useful in the eventual computation to be performed on this data that the data be serial by trace; that is, it is desirable that all the data from a given trace be stored on magnetic tape first, then all the data from a second trace, and so on. Therefore on-board processing means are provided to re-format this data into a second format in which it is desired to be presented to a land based computer.
It is desirable that the operation of these reformatting means, which may comprise a plurality of computers, as well as interface means and other well known data processing means, be monitored in real-time; that is, while the reformatting is going on. The present invention is directed at this point and comprises real-time monitor means for producing a visual record on paper (i.e. a hard copy) of the data as it is stored on magnetic tape in the form in which it is desired to present it to a land based computer for analysis and study.
It will be appreciated that a vast quantity of data is generated in the process as described above. Indeed, an entire reel of magnetic tape is completely filled with data generated in this fashion in approximately eleven minutes according to the system as described above. Likewise, it would be appreciated that a similarly vast amount of data must be monitored by the monitor means. Therefore, it is desirable to provide means for making this data more clearly interpretable by the eye so that any errors in the recording process could be better detected. Any deficiencies of the display means chosen to show the real-time picture of the data being stored should be overcome. For example, traces of the analog wave forms produced at the geophones appear as sinusoidal type waves, although, of course, not of perfect sine wave shape. If a large quantity of these waves are reproduced on a single sheet of paper by a monitor plotter, they tend to be rather small and indistinct from one another, since so much data is being compressed and displayed on a single sheet of paper. It has been found by applicants that by partially overlapping successive traces and filling-in the positive portion of the graphed wave shape, the data is made much more clearly readable, and errors are significantly better detectable. A similar situation would prevail if the data were being displayed on a cathode ray tube such as a TV or oscilloscope screen, rather than on a piece of paper, and this discovery has utility there as well.
Clearly, it would be advantageous to provide monitor means at reasonable cost and of high efficiency. Specifically, it is desirable that the computer used to generate the display be similar to those used to reformat the data, so that similar software may be employed and so that the monitor computer can serve as a back-up to the reformatting computer(s).